Spark plugs including an ignition chamber have been developed. For example, a pre-chamber ignition plug according to Japanese Unexamined Patent Application Publication No. 2012-199236 (“PTL 1”) includes a cylindrical metal housing, and an ignition chamber cap that surrounds a center electrode and a ground electrode to form an ignition chamber. The ignition chamber cap has multiple orifices that allow an air-fuel mixture to flow into the ignition chamber from a combustion chamber. This ignition plug ignites in the ignition chamber, and injects torch-shaped flames into the combustion chamber through the orifices to burn an air-fuel mixture in the combustion chamber.
The ignition plug disclosed in PTL 1, however, has a structure where the ignition chamber is closed except for the orifices. Thus, the temperature inside the ignition chamber tends to rise at the ignition, which may cause pre-ignition. On the other hand, when the temperature inside the ignition chamber is excessively lowered, pressure loss and heat loss increase during combustion inside the ignition chamber, so that pressure and heat quantity of the injection into the main combustion chamber decrease, which may cause misfires. Therefore, a configuration that can suppress pre-ignition and misfires has been desired by setting thermal conductivity and the volume to appropriate values in a housing and an ignition chamber cap that significantly affect heat conduction in the ignition chamber.